Technical Field
The present invention relates to an adsorption characteristic measuring apparatus, and more particularly to an adsorption characteristic measuring apparatus for obtaining an adsorption isotherm using a constant volume method.
Related Art
As a method of evaluating pore distribution in a material or a specific surface area of the material, a gas adsorption method is used. For example, an adsorption characteristic measuring apparatus, which measures an adsorption isotherm using a constant volume method to evaluate a specific surface area or a pore distribution, includes a plurality of pressure gauges, valves, and a vacuum exhaust system to supply a predetermined number of moles of adsorbate to a sample in the sample tube, which is set to constant volume and temperature, and to measure the pressure change before and after adsorption.
JP 2014-81250A discloses an example where a powder sample is accommodated in a sample tube made of glass, a nitrogen gas is used as an adsorbate, the sample tube is disposed in a Dewar vessel filled with liquid nitrogen, and a pressure change in the sample tube is measured under a constant liquid nitrogen temperature of 77 K.
As a technique related to the invention, JP 2008-247702A discloses a method of producing a zeolite film, which is formed on a surface of a porous tubular support body made of ceramic such as alumina, as a separation film by which carbon monoxide is selectively separated from a hydrocarbon gas.
A zeolite film formed body disclosed in JP 2008-247702A is configured such that a thin zeolite film is formed on the surface of the support body and is used in a composite structure where the support body and the zeolite film are integrated. The sample object used for the measurement of adsorption characteristics is generally in the form of a powder or the like. However, since the zeolite film formed body has the integrated composite structure, it is meaningless to try to measure the adsorption characteristics even though using powder of the zeolite film and the support body in a mixed state. Each of the zeolite film and the support body can be separately powdered to measure adsorption characteristics, but it is not possible to evaluate the adsorption characteristics of the zeolite film formed body. Therefore, in the case of trying to measure the adsorption characteristics in an intact form of the zeolite film formed body, the size of the sample tube becomes larger, a dead volume increases, and measurement accuracy of the adsorption characteristics is lowered.
Furthermore, in the zeolite film formed body, zeolite crystals having micropores are aggregated in an amorphous form and an inter-crystal void has a size of mesopores or macropores. Here, the micropore refers to a pore having a diameter of less than 2 nm, the mesopore refers to a pore having a diameter from 2 nm to 50 nm, and the macropore refers to a pore having a diameter of larger than 50 nm. As the diameter of the pores becomes smaller, an adsorption layer of an adsorption gas becomes thinner and discrete adsorption proceeds. In addition, since heat of adsorption is generated at the time of the adsorption, there is a problem that adsorption equilibrium is difficult to attain. In order to measure the adsorption amount of gas molecules which are discretely and thinly adsorbed using a constant volume method, it is necessary to measure an adsorption isotherm with high accuracy under very low relative pressure. The adsorption gas is lean under the very low relative pressure and has a thermal conductivity lower than that of an adsorption gas having higher pressure, and thus it is difficult to maintain the sample, which passes through the sample tube from a refrigerant tank used to maintain an isothermal state, at a predetermined constant temperature. Herein, the relative pressure is a pressure of the adsorption gas normalized by a saturated vapor pressure of the adsorption gas and has a dimensionless value.
In order to obtain the adsorption isotherm of the film formed body with high accuracy as described above, there are problems in that the measurement should be performed with high accuracy under the very low relative pressure, so that the influence of the heat distribution in the sample tube is eliminated. Until now, measuring the adsorption isotherm of the film formed body itself with high accuracy has not been performed.